1. Field of Invention
This invention relates to highly sensitive strain sensor devices made from encapsulated core-shell nanofiber meshes. The sensor films can be adhered to or embedded in a variety of objects and textiles for monitoring of deformation, pressure and vibrations for tactile, structural health, living object health, and vital sign monitoring with accuracy, low cost and easy read out.
2. Description of Related Art
Natural systems, such as human skin, are covered with sensors of mechanical deformation or pressure to provide the “sense of touch”. An electronic skin that can provide tactile sensing to synthetic products has a wide range of applications, including measurement of mechanical strain and cracks for monitoring of aging infrastructure (such as buildings, skyscrapers, bridges, dams, pipelines, turbine blades and drain system), sensing mechanical forces on composite body of aviation or other transportation vehicles for structural health monitoring and safety, vibrational sensors for monitoring operation of complex machinery, and touch sensors for human-media devices such as e-books and smart/mobile phones. In addition, by attaching such devices to human body or clothing in form of implantable or wearable electronics, vital health signs such as heartbeat, breathing, muscle movements and blood pressure can be monitored through smart wound coverings or smart fabric/clothing.
Conventional strain and pressure sensors use piezoelectric ceramics (e.g., lead zirconate titanate (PZT)), piezoelectric polymers (e.g., polyvinylidene fluoride (PVDF)), pyroelectric ceramics (e.g., barium titanate (BaTiO3) and piezoresistive semisonductors (e.g., polysilicon). The sensors provide a gauge factor ranging from 0.0001 to 30. These materials are costly, bulky, can have toxic elements such as lead, and have complex deposition and post processing processes. Commercial PZT sensors are costly, rigid and brittle, toxic and environmental unfriendly, and are hard to manufacture over large area and at low cost. Metallic strain gauges and piezoresistive sensors have smaller gauge factors and small threshold for sensitivity. Strain sensors based on aligned and random meshes of nanowires (NWs), carbon nanotubes (CNTs) and CNT composites have been demonstrated but suffer from large area scalability, reproducibility of response and low gauge factors in the range of 0.0001-2.
Some patents that describe the current state of the art for systems for low cost strain sensors and sensors for electronic skin are as follows: U.S. Pat. No. 8,159,235 (Lynch et al., 2012) have shown electrical impedance tomography of nanoengineered thin films. U.S. Pat. No. 8,108,157 (Chase et al., 2012) have shown electrospun fibrous nanocomposites as permeable, flexible strain sensors. These sensors made of conductive fibres whose resistance change due to narrowing of the current path between two electrodes by stretching. The sensitivity, low resistance and repeatability of encapsulated nanofiber mesh presented in this invention is significantly higher that allows applications for wearable electronics and structural health monitoring not useful in the prior art. U.S. Pat. No. 7,921,727 (Rice, 2011) have shown sensing system for monitoring the structural health of composite structures.